Compensated directional coupler



May Z, 1967 E. sALzBL-:RG

COMPENSATED DIRECTIONAL COUPLER Filed Deo. 28, 1964 com pensoted oonvenfionol @E538 @Eg Frequency F I G. 2

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INVENTOR. EDWARD SALZBERG BY ww n i FIGQ ATTO R N EYS United States Patent llice 3,317,859 CGMPENSATED DHRECTINAL COUPLER Edward Sahberg, Wayland, Mass., assignor to Microwave Development Laboratories, Inc., Needham Heights, Mass., a corporation of Massachusetts Filed Bec. 28, 1964, Ser. No. 421,325 3 Claims. (Cl. S33-10) This invention relates generally to electromagnetic Wave transmission apparatus and more particularly, the invention pertains to an improved directional coupler pro viding substantially fiat coupling over the couplers operational range.

A conventional directional coupler consists of a main signal channel coupled to an auxiliary signal channel in a manner such that wave energy propagating in one direction in the main signal channel is partially transferred to the auxiliary signal channel and is transmitted in one direction only in the auxiliary channel. A directional coupler, thus, is a device that responds to wave energy propagating in one direction by transmitting to an output a fraction of that wave energy.

In many situations where directional couplers are ernployed, it is desired that the fraction -of power be precise and that the fraction be unvarying despite the range of frequencies which the input signal may have. Conven-f tional waveguide directional couplers exhibit a variation in coupling as a function of signal frequency. That is, the amount of coupling provided by a conventional waveguide directional coupler will vary with the frequency of the signal applied to the couplers input. That variation in coupling of the conventional waveguide directional cou pler is principally attributable to the characteristics of the coupling apertures which permit the transfer of wave energy between the couplers two signal channels. The primary objective of the invention is the provision of a directional coupler whose variation in coupling as a func- -tion of signal frequency is materially reduced so as to obtain substantially hat coupling over the couplers entire operational range.

The invention contemplates an arrangement of apparatus to provide compensation for directional couplers exhibiting frequency sensitive coupling. In the arrangement, two directional couplers are connected in tandem in a manner such that the phase of the signal in one channel is shifted relative to the signal in the other channel by an amount dependent upon the signal frequency. By that arrangement, the frequency sensitive coupling of one of the directional couplers is compensated by yappropriately phased wave energy from the other directional coupler in a manner causing the coupling of the apparatus to be substantially flat over its entire operational range.

The invention, both as to its arrangement and manner of operation, can be better understood from the following exposition when considered in connection with the ac- -companying drawings in which:

FIG. l depicts a conventional broad wall directional coupler;

FIG. 2 is a graph illustrating the manner in which the coupling of the conventional directional coupler varies with signal frequency;

FIG. 3 shows a preferred embodiment of the invention;

FIG. 4 is a graph showing the improved performance of the preferred embodiment; and

FIG. 5 is a diagrammatic symbolic representation of the invention.

Referring now to the conventional broad wall directional coupler depicted in FIG. 1, it is apparent that the coupler essentially consists of two hollow rectangular waveguides 1 and 2 having a common broad wall 3 in Patented May 2, 1967 which there are four circular apertures 4, 5, 6, and 7. The outer two apertures are of smaller diameter than the inner pair. As is usual, the circular apertures are spaced at )cg/4 intervals, where xg is the mean wavelength in the guide of the couplers operational frequencies. The term operational frequencies as used in this exposition denotes the band of frequencies over which the directional coupler is required to perform satisfactorily.

Assuming that waveguide 1 is the primary channel, waveguide 2 the auxiliary channel, and that wave energy is launched into the primary channel so that it propagates along the guide in the direction of the arrow 9, a portion of the wave energy in the signal couples through each aperture into the auxiliary channel. Four components of wave energy, therefore, enter the auxiliary channel through the -coupling holes and the spacing of the holes causes the phases of those components to be such that the wave energies are additive when propagating in the direction of arrow 8 but cancel when propagating in the opposite direction. The wave energy coupled into the auxiliary channel, therefore, can flow only in the direction of arrow 8 and the amount of that energy is proportional to the power flowing in the main channel in the direction of arrow 9.

In the ideal directional coupler the fraction of wave energy flowing in the auxiliary channel is invariable over the couplers operational range. That is, the ideal coupling isflat over the entire frequency band as indicated by the broken horizontal line in FIG. 2 where frequency is plotted along the abscissa and the relative coupling in decibels (db) is plotted along the ordinate. In the conventional broad wall multihole coupler, the coupling is tightest at the ends of the band with the loosest coupling occurring somewhere about the bands center, as indicated by the conventional full line in FIG. 2. As a compromise, the conventional coupler is usually constructed so that its departure from the ideal coupling characteristic is minimized by having its characteristic curve extend equally above and below the ideal flat coupling line, as shown in FIG. 2. The variation in coupling as a function of signal frequency of theconventional multihole coupler is principally due to the characteristics of the apertures which couple energy from the primary channel to the auxiliary channel. When the coupling is tight a higher proportion of energy is transferred than is trans-y ferred when the coupling is loose In most conven tional directional couplers, the coupling is tightest at the edges of the band, although there is a variety of conventional coupler, known as the cross-guide coupler in which the coupling is tightest at the bands center and loosest at the edges.

For expository purposes, the invention is described in relation to a conventional multihole coupler in which the coupling is tightest at the band edges. FIG. 3 depicts a preferred embodiment of the invention employing two broad wall multihole directional couplers 10 and 11. The primary channels of the two directional couplers are connected by a straight length of waveguide 12 of path length l1 whereas the auxiliary channels of the two directional couplers are connected by a frequency sensitive phase shift device in the form of a waveguide loop 13 of path length l2. The difference in electrical degrees of path length l2 with respect to path length Z1 is denoted by 0. The frequency dependence `of coupling of the couplers 10 and 11 are, preferably, alike so that the coupling characteristic curves for both couplers, for example, may be of the general contour of the conventional curve in FIG. 2. The wave energy coupled through coupler 11, when correctly phased with respect to the energy coupled through coupler 1f), is of such amount as to compensate for the frequency variation in coupling of coupler 1f). For example, assuming coupler 10 to be a 3db coupler, then coupler 11, may for example, be a 25db or 30d-b coupler. That is, the directional coupler which introduces the compensating wave energy must couple just enough energy to correct the departures from the ideal of the other coupler. If improperly selected, the compensating coupler may overcompensate or undercornpensate the other coupler.

Assuming that coupler Il provides the corre-ct amount of coupled energy, H is chosen so that the phase shift introduced by waveguide loop 13 causes the energies coupled into the auxiliary channel by couplers 16 and 11 to be additive at the center of the frequency band and subtractive at the band edges. The total coupling of the apparatus as a function of frequency, therefore, is as indicated by curve 14 of FIG. 4, close to the ideal characteristic.

The invention is diagrammatically shown in FIG. 5 where line A represents the connected primary channel, line B represents the connected auxiliary channel, the phase shifter is indicated as a loop in the auxiliary channel, coupler is represented by the coupling coefficient K1, and coupler 11 is represented by the coupling coefficient K2. Kt designates the coupling coefficient of the entire apparatus. In the following discussion, all the Ks are coupling coecients expressed in power ratio.

On applying a signal at the input, it can be seen that Kt will be constituted by two components, one component Ibeing the coupling K1 diminished by its attenuation at coupler 11, and the other component being the coupling K2 diminished by attenuation due to the transfer at coupler 10 of some of the wave energy initially in line A. The resultant, Kt, is also related to the phase angle 0 between the voltages of the two components. Expressed mathematically,

To find the absolute value of Kt resort is had to the law of cosines,

C2=A2+B22AB cos a Substituting, the expression is therefore and the phase shift at the high frequency edge of the band is expressed by In both of the above expressions d is the excess length of line B with respect to line A xgH is the wavelength in the guide at the high end of the frequency band Agr, is the wavelength in the guide at the low end of the frequency band.

It is desired to obtain the condition where the cosine of 0L equals the cosine of 0H, viz.,

By algebraic manipulation, therefore, the excess line length d is From a comparison of the compensated coupling characteristic curve 14 shown in FIG. 4 with the conventional coupling curve shown in FIG. 2, it can be seen that the maximum original deviation of .Sdb has been reduced to a maximum deviation of about .1db.

For ease of exposition, coupler 10 was assumed to have a frequency variation of coupling that is substantially symmetrical about the center of the band so that the departure from the ideal is equal at the band edges. It was also expressly assumed that coupler 1I has a frequency variation of coupling that is generally the same as that of coupler 10. In practice, those conditions can be closely realized, but the invention is not restricted to those conditions. The characteristic curve of the variation of coupling as a function of frequency of coupler 10, determines to a large extent the characteristic curve that coupler il should have. The characteristic curve of coupler 10 need not be symmetrical and can have forms distinctly different from the conventional curve of FIG. 2. By careful choices of a characteristic curve for coupler 11 and the phase shift 0 some degree of compensation can be achieved -for almost any directional coupler.

In the preferred embodiment, the phase shift was obtained by employing a waveguide loop. To those knowledgeable in the art of electromagnetic wave transmission, it is obvious that the phase shift can be obtained by a change in the width of the waveguide, by inserting a length of dielectric material in one or both channels, or by using a series of reflective irises in the waveguide. While the phase shifter can be any of a number of different devices, its arrangement in the apparatus is a part of the invention.

The invention has been described in connection with a rectangular waveguide directional coupler. While the preferred embodiment is illustrated as broad wall multihole coupler, the invention is applicable also to rectangular waveguide side wall couplers. Further, directional couplers constructed of other types of waveguide such as ridged and round guide, couplers constructed of coaxial line or strip transmission line, and even lumped constant couplers can be compensated Iby using adaptations of the arrangement constituting the invention.

In consideration of the numerous ways in which the invention can be embodied, it is intended that the invention not be restricted to the illustrated embodiment, but rather that the scope of the invention be construed in accordance with the appended claims.

What is claimed is:

1. Apparatus for transmittino a band of signal frequencies, the apparatus comprising:

(l) a first directional coupler;

(2) a second directional coupler providing substantially looser coupling than the first coupler, the second directional coupler compensating the output of the apparatus for the frequency variation in coupling of the first directional coupler;

(3) and a phase shifter connected to transmit wave energy from one coupler to the other, the phase shifter causing the phase of Wave energy delivered to the primary channel of the receiving coupler to be shifted as a function of signal frequency relative to the phase of Wave energy delivered to the auxiliary channel of the receiving coupler, whereby the frequency variation in coupling of the first directional coupler is compensated by the coupling of the second directional coupler.

2. Apparatus for transmitting a band of signal fre quencies, the apparatus comprising:

(l) a first directional coupler exhibiting a frequency 5 variation in coupling within the signal frequency band;

(2) a second directional coupler having a frequency variation of coupling Within the signal frequency band, the second directional coupler providing looser coupling than the first coupler, the magnitude of coupling of the second coupler relative to the magnitude of coupling of the rst coupler being such as to compensate for the frequency variation in coupling of the rst coupler;

(3) and a phase shifter connected t-o transmit wave energy from one coupler to the other, the phase shifter ,causing the phase of wave energy delivered to the primary channel of the receiving coupler to be shifted as a function of frequency relative to the wave energy delivered to the receiving couplers auxiliary channel, whereby the frequency variation in coupling of the rst directional coupler is compensated by the coupling of the second coupler.

3. Microwave apparatus for transmitting a band of sig nal frequencies, the apparatus comprising:

(l) a first waveguide coupler;

(2) a second waveguide coupler providing substantially looser coupling than the rst coupler;

(3) a first length of waveguide connecting the primary channels of the two couplers;

(4) and a second length of waveguide connecting the auxiliary channels of the two couplers, the two connecting waveguides differing in length by an amount UNITED STATES PATENTS 2,679,631 5/1954 Korman et al. S33-31 X 2,849,685 8/1958 Weiss 333-11 3,034,076 5/1962 Tomiyasu 333-10 X OTHER REFERENCES L. Sweet: A Method of Improving the Response of 20 Waveguide Directional Couplers, IEEE Transactions on Microwave Theory and Techniques, vol. MTT-ll No. 6, November 1963, p. 554.

HERMAN KARL SAALBACH, Primary Examiner. M. NUSSBAUM, Assistant Examiner. 

1. APPARATUS FOR TRANSMITTING A BAND OF SIGNAL FREQUENCIES, THE APPARATUS COMPRISING: (1) A FIRST DIRECTIONAL COUPLER; (2) A SECOND DIRECTIONAL COUPLER PROVIDING SUBSTANTIALLY LOOSER COUPLING THAN THE FIRST COUPLER, THE SECOND DIRECTIONAL COUPLER COMPENSATING THE OUTPUT OF THE APPARATUS FOR THE FREQUENCY VARIATION IN COUPLING OF THE FIRST DIRECTIONAL COUPLER; (3) AND A PHASE SHIFTER CONNECTED TO TRANSMIT WAVE ENERGY FROM ONE COUPLER TO THE OTHER, THE PHASE SHIFTER CAUSING THE PHASE OF WAVE ENERGY DELIVERED TO THE PRIMARY CHANNEL OF THE RECEIVING COUPLER TO BE SHIFTED AS A FUNCTION OF SIGNAL FREQUENCY RELATIVE TO THE PHASE OF WAVE ENERGY DELIVERED TO THE AUXILIARY CHANNEL OF THE RECEIVING COUPLER, WHEREBY THE FREQUENCY VARIATION IN COUPLING OF THE FIRST DIRECTIONAL COUPLER IS COMPENSATED BY THE COUPLING OF THE SECOND DIRECTIONAL COUPLER. 